[S-CG40] Science of slow-to-fast earthquakes

[SCG40-P12] Optimal transport-based full waveform inversion for high resolution imaging of plate boundary structures in the slow earthquake gap off Western Kii Peninsula, Nankai Trough

*Yanfang Qin¹, Gaku Kimura¹, Gou Fujie¹, Shuichi Kodaira¹, Yasuyuki Nakamura¹, Seiichi Miura¹ (1.Japan Agency for Marine-Earth Science and Technology)

Slow earthquakes are crucial for understanding the nature of subducting plate boundaries and their seismic coupling. In the Nankai Subduction Zone, numerous slow earthquakes have been observed, and their distribution exhibits along-strike variation. Although many very low-frequency earthquakes (VLFEs) have been recently observed in the central part of the Nankai Trough, there have been only rare cases reported between the Kii Channel and Cape Shionomisaki. To investigate the structural features and their potential relationship to the slow events, in November 2018, JAMSTEC deployed a dense array of 96 ocean-bottom seismometers (OBS) to acquire an active-source seismic refraction dataset from the seaward side of the subduction trough to the accretionary prism off Cape Shionomisaki. This type of wide-angle seismic refraction data is widely used for imaging the lithosphere using a ray trace-based method, which fails to detect small-scale heterogeneities in subsurface structures. Full-waveform inversion (FWI) utilizes the amplitude and phase information available in the data to provide high-resolution models of elastic properties and is well-suited for detecting small-scale structures. In this study, we apply graph-space optimal transport based FWI to the OBS data in time domain. This approach has the potential to provide a more convex misfit function than the conventional least-squares distance misfit function, mitigating cycle skipping issue. The reconstructed P-wave velocity model illuminates highly complex geological structures. Additionally, we coordinated the data with a seismic reflection profile to interpret subsurface structural features. Among the many newly discovered features in the results, this new velocity model accurately characterizes the shapes of intrusion magmatic bodies in the forearc region, which may contribute to the formation of a strong inner wedge. The outer wedge appears weak and contains some low-velocity anomaly zones, which possibly link to specific deformation processes and the evolution of the accretionary prism. Furthermore, we conducted a comparison between our profile and a transect across the adjacent Kumano Basin, a region with dense VLFEs. The distinct results from these two locations reveal structural and physical differences that cause variability in seismic activities.